The present invention relates to a frequency offset compensation system, and in particular relates to such a system in a digital satellite communication system which is used in FDMA (frequency division multiple access) using narrow band modulation signal.
The frequency stability of a local oscillator for a satellite communication system at present is typically in the range from .+-.1.times.10.sup.-5, to .+-.1.times.10.sup.-6. In a digital mobile satellite communication system in which C-band (4 GHz band for return link, and 6 GHz band for forward link) is used between a land earth station and a satellite, and L-band (1.5 GHz band for return link and 1.6 GHz band for forward link) is used between a mobile earth station and a satellite are used, considerably large frequency deviation up to 50 kHz-500 kHz occurs both in a C-L link and an L-C link. Further, when an inclination of an orbit of a satellite is large, a doppler shift occurs due to a figure 8-shaped movement of a satellite, and the doppler shift would be 500 Hz when the inclination of the orbit is 5.degree..
Therefore, a conventional receiver in an earth station has AFC (automatic frequency control) in a C-L link and an L-C link, since it is impossible to directly demodulate a receive signal which is subject to large frequency error, and an adjacent channel interference must be taken into account in an FDMA system.
FIGS. 4 and 5 show a conventional mobile satellite communication system using AFC.
FIG. 4 illustrates a configuration used for a conventional C-L link, and has a closed loop control system using an AFC pilot signal 6 (6-1, 6-2) for controlling transmit frequency of a land earth station 9. The AFC pilot signal 6-1 in a C band transmitted by the land earth station 9 is received by the same station 9 in an L band through the frequency conversion in the satellite 1. Then, the received frequency is compared with reference frequency of a pilot signal receiver in the land earth station. The frequency offset is fed back to a local oscillator of a C band frequency converter in the land earth station so that the frequency offset is compensated.
FIG. 5 illustrates a configuration which is used for a conventional L-C link, and has an open loop control system using an AFC pilot signal 6-2 for controlling local frequency in a receiver in a land earth station. An AFC pilot signal in an L band transmitted by the land earth station is received by the same land earth station in a C band through frequency conversion in the satellite 1. Then, the received frequency is compared with reference frequency of a pilot signal receiver in the land earth station. The frequency offset is fed back to a local oscillator of a C band receiver in the land earth station so that the frequency offset is compensated.
However, the amount of doppler shift for AFC pilot signal due to figure 8-shaped movement of a satellite depends upon whether it is a forward link (from a land station 9 to a satellite 1) or a return link (from a satellite 1 to a land station 9). Therefore, it is impossible to separate doppler shift in a forward link from that of a return link. Therefore, communication must be influenced by doppler shift both in a forward link (C-L link) and a return link (L-C link).
In order to solve the above problem, an EAFC (Enhanced AFC) has been proposed in a paper entitled "Frequency Control of Narrowband Digital Carriers in the Presence of Satellite Doppler and Other Disturbances", in proceedings in 9th international conference on digital satellite communications, pages 213-220, May 18-22, 1992, organized by INTELSAT. The EAFC system is shown in FIGS. 6 and 7. The EAFC installs a reference earth station 10 separated from a land earth station 9, for providing a pilot signal (12-1, 12-2). The reference earth station 10 estimates an amount of doppler shift based upon location information of a land earth station and a reference earth station, orbit information of a satellite, and received data of pilot signal for 24 hours, so that the amount of doppler shift in communication channel between a land earth station and a satellite is compensated. The control for EAFC pilot signal for a forward link is carried out independent from that for a return link, since frequency in a forward link differs from frequency in a return link. So, EAFC pilot signal 12-1 is controlled so that doppler shift in a return link from a satellite to a land earth station is compensated, and EAFC pilot signal 12-2 is controlled so that doppler shift in a forward link from a land earth station to a satellite is compensated.
However, the EAFC system has the disadvantage that only the doppler shift between a satellite and a land earth station, but the doppler shift between a satellite and a mobile earth station is not compensated. Therefore, the doppler error up to 500-600 Hz exists in spite of the EAFC.
Typically oscillator in a land earth station has long term stability of around .+-.1.times.10.sup.-8. However, an oscillator in a mobile station typically has a long term stability around .+-.1.times.10.sup.-5. Therefore, signal quality of the receive signal in a mobile station is significantly deteriorated due to frequency shift which is caused by unstability of an oscillator in a mobile station. In order to solve that problem, conventionally, a mobile station takes a frequency offset compensation system by synchronizing a transmit frequency of the mobile station with a forward link receive signal frequency transmitted from a land earth station.
However, even when such a frequency offset compensation system is used with the combination of said AFC system or said EAFC system, it has the disadvantage that a land earth station must demodulate a receive signal which has significant frequency error generated in both a forward link and a return link between a satellite and a land earth station, and doppler shift between a satellite and a mobile station. This causes significant difficulty to a land earth station, and complicates the structure of the land earth station.
Further, in an FDMA system in which a modulation signal is in the narrow band, signal quality is deteriorated due to adjacent channel interference, since frequency offset compensation error of AFC or EAFC in each channel differs from a frequency offset due to doppler shift between a satellite and a mobile station.
No suitable solution for compensating that adjacent channel interference has been proposed. Therefore, a current digital mobile satellite communication system requires wide guard band between channels. However, these systems have the disadvantage that the efficiency of frequency use in a transponder in a satellite is significantly decreased.
Therefore, it has been a goal to develop a frequency offset compensation system wherein efficiency of frequency use in a transponder in a satellite is increased.